1. Field of the Invention
The invention is related to the field of nuclear magnetic resonance exploration of oil wellbore. In particular, the invention is a method of correcting for ringing effects inherent in magnetic pulse sequence testing.
2. Description of the Related Art
A variety of techniques are utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, the resistivity, porosity and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, the tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling, which is referred to as measurement-while-drilling (MWD) or logging-while-drilling (LWD).
One recently evolving technique involves utilizing Nuclear Magnetic Resonance (NMR) logging tools and methods for determining, among other things, porosity, hydrocarbon saturation and permeability of the rock formations. The NMR logging tools are utilized to excite the nuclei of the liquids in the geological formations surrounding the wellbore so that certain parameters such as spin density, longitudinal relaxation time (generally referred to in the art as T1) and transverse relaxation time (generally referred to as T2) of the geological formations can be measured. From such measurements, porosity, permeability and hydrocarbon saturation are determined, which provides valuable information about the make-up of the geological formations and the amount of extractable hydrocarbons.
The NMR tools generate a uniform or near uniform static magnetic field in a region of interest surrounding the wellbore. NMR is based on the fact that the nuclei of many elements have angular momentum (spin) and a magnetic moment. The nuclei have a characteristic Larmor resonant frequency related to the magnitude of the magnetic field in their locality. Over time the nuclear spins align themselves along an externally applied magnetic field. This equilibrium situation can be disturbed by a pulse of an oscillating magnetic field, which tips the spins with resonant frequency within the bandwidth of the oscillating magnetic field away from the static field direction. The angle θ through which the spins exactly on resonance are tipped is given by the equation:θ=γB1tp.  (1)where γ is the gyromagnetic ratio, B1 is the effective field strength of the oscillating field and tp is the duration of the RF pulse.
After tipping, the spins precess around the static field at a particular frequency known as the Larmor frequency ω0, given byω=γB0  (2)where B0 is the static field intensity. At the same time, the spins return to the equilibrium direction (i.e., aligned with the static field) according to an exponential decay time known as the spin-lattice relaxation time, or longitudinal relaxation time, T1. For hydrogen nuclei, γ/2π=4258 Hz/Gauss, so that a static field of 235 Gauss would produce a precession frequency of 1 MHz. T1 of fluid in pores is controlled totally by the molecular environment and is typically ten to one thousand milliseconds in rocks.
Typically, measurement of NMR related phenomena in the earth formation is performed by allowing some time for the static magnetic field to polarize nuclei in the formation in a direction substantially along the direction of the static magnetic field. A first one of the RF pulses passed through the antenna has a magnitude and duration selected to reorient the nuclear magnetization by about 90 degrees from its previous orientation. This pulse is referred to in the prior art as the A-pulse, the 90°-pulse, and the excitation pulse, among others. After a selected time, successive RF pulses are passed through the antenna, each of these having a magnitude and duration selected to reorient the nuclear spin axes by about 180 degrees from their immediately previous orientations in order to enable the nuclear spin axes to “rephase” or realign with each other. These pulse rephrasing pulses are referred to in the prior art as the B-pulses, the 180°-pulses, and refocusing pulses, among others. The induced signals, known as “spin echoes”, are generally measured during the time interval between each successive one of the “180 degree” RF pulses. The succession of spin echo measurements is generally known as a “sequence”. The amplitude of the spin echo signals, and the rate at which the spin echo amplitudes change during a measurement sequence, are related to properties of interest of the earth formations, such as fractional volume of pore space (porosity) and the properties of fluids present in the pore spaces. The frequency of the RF magnetic field needed to reorient the nuclear magnetization, which is the frequency of the spin echo signals, is related to the amplitude of the static magnetic field and a factor, known as the gyromagnetic ratio γ, which is unique to each isotope. For evaluation of earth formations, the static magnetic field amplitude and RF magnetic field frequency are typically selected to excite NMR phenomena in hydrogen nuclei, although other nuclei may be used for NMR evaluation of earth formations.
Exciting the antenna with RF power pulses in the presence of a strong static magnetic field causes mechanical excitation of the antenna. Mechanical excitation of the antenna leads to excitation of a signal, called “ringing”, in the antenna. The ringing is unrelated to NMR phenomena, and frequently has a very large amplitude. The amplitude of the ringing is often highest right after application of each RF pulse, and is of such a magnitude as to make it difficult to measure the amplitude of NMR induced signals. Reducing the effects of ringing on NMR measurement is very important in well logging applications, among others, because significant information about the properties of the earth formations are determined by the amplitudes of spin echoes occurring shortly after the RF pulses.
Apparatuses which reduce ringing effects by way of tool design are discussed, for example in U.K. Patent Publication GB2310724A, of Taicher, and U.S. Patent Publication 20030038631 of Kruspe. Methods for removing the effects of acoustic ringing in Nuclear Quadrupole Resonance techniques are discussed in U.S. Pat. No. 5,365,171, of Beuss.
Several methods are known in the art for removing ringing effects. One class of such methods includes creating a phase difference between the ringing signal and the NMR induced signals, and summing or “stacking” multiple sets of measurements to reduce the amplitude of the ringing signal in the output. One commonly used measurement sequence used in evaluation of earth formations is known as “phase alternate pairs” (PAPS). PAPS sequences include performing a measurement sequence as just described including a 90 degree RF pulse followed by successive 180 degree pulses. After a selected wait time, another such measurement sequence is performed, but with the polarity of the 90 degree pulse reversed. Stacking the two sets of measurements substantially cancels the ringing signal. Such a method is described, for example, in U.S. Pat. No. 5,596,274 to Sezginer and U.S. Pat. No. 5,023,551 to Kleinberg et al.
In traditional PAPS techniques, first an acquisition is performed at time t and the resulting measurement, m(t) contain both useful echo data, e(t), and corrupting ringing data, r(t), resulting inm(t)=e(t)+r(t).  (4)After this acquisition, it is typically necessary to wait a time for the measured material to recover, leading to a long repetition rate, trl. The acquisition is repeated, this time with identical parameters, except that the phase of the excitation pulse is moved through 180°. This has the effect of changing the sign of the echo data, but it does not affect the ringing which is primarily created by the refocusing pulses. So:m(t+trl)=−e(t+trl)+r(t+trl).  (5)The PAP technique is to subtract these two measurements, i.e.
                              PAPS          ⁡                      (                          t              +                                                tr                  l                                2                                      )                          =                                                            m                ⁡                                  (                  t                  )                                            -                              m                ⁡                                  (                                      t                    +                                          tr                      l                                                        )                                                      2                    =                                                                      e                  ⁡                                      (                    t                    )                                                  +                                  e                  ⁡                                      (                                          t                      +                                              tr                        l                                                              )                                                              2                        +                                                            r                  ⁡                                      (                    t                    )                                                  -                                  r                  ⁡                                      (                                          t                      +                                              tr                        l                                                              )                                                              2                                                          (        6        )            If the ringing is constant or very slowly changing, the PAPS result is the average of the two echo signals and contains little or no ringing signal. Usually, the material that is ringing is within the instrument (sensor or electronics) and the assumption that the ringing changes slowly with time. However, in some circumstances, e.g. metal debris in the well, the ringing material may be external to the instrument. This external material typically moves with respect to the instrument and therefore changes the ringing during time interval between the pulse sequences. The duration of time between the first and second sequences of the PAPS sequence must necessarily be large to allow for adequate spin-lattice relaxation before the second sequence. The extended length of time also leads to different material being tested in each sequence of the PAPS because of the motion of the measurement device through the borehole. The wait time, trl, thereby limits the effectiveness of the results. Another disadvantage of the PAPS sequence is that it only eliminates ringing due to the application of the B-pulses, and does not reduce ringing from the A-pulse.
Several other methods for reducing the effects of ringing are known in the prior art. U.S. Pat. No. 6,498,484 to Sun et al., discusses a method in which two CPMG pulse sequences are used with variation of the static magnetic field. In the second pulse sequence, the static field amplitude is adjusted by an amplitude and time span selected to cause a 180 deg phase shift in the spin echo signals. Base line noise and ringing substantially cancels when the first and second signals are subtracted. In U.S. Pat. No. 6,121,774 to Sun et al., during a first time period of a single pulse sequence cycle, a first plurality of oscillating pulses is applied to a volume of formation. The subsequent measured signal comprises a ringing component and a plurality of spin-echoes. During a second time period of the single pulse sequence cycle, a second plurality of oscillating pulses are applied to the volume of formation and signals generated in the formation are measured. The measured signals comprise the ringing component and substantially exclude the spin-echoes. During the second time period, the spin-echoes and stimulated echoes may be eliminated by repeatedly applying a short pulse followed by a time delay in order to spoil the stimulated and echoes and spin-echoes. The signals measured during the first time period are corrected to eliminate the ringing component.
U.S. Pat. No. 6,377,042 to Menger et al., discusses a method of obtaining enhanced-resolution NMR data by merging, in the time domain, different NMR pulse echo trains into a single echo train. The input echo trains can be acquired with different inter-echo spacing, wait time, and signal-to-noise ratio parameters that are optimized to correspond to both fast and slow portions of the T2 spectrum. The merged echo trains are inverted into complete T2 spectra in a single step, the merged echo train typically carrying information about both relatively fast and relatively slow NMR signals.
In U.S. Pat. No. 6,541,969 to Sigal et al. an estimate of the ringing component of the signal can be obtained by combining two or more acquisition sequences in such a manner as to obtain an estimate of the ringing component of the signal. Alternatively, the ringing component of the signal is estimated by direct measurement using a separate NMR pulse sequence, which is a specific implementation is a standard CPMG pulse echo sequence without the leading 90° pulse. Such sequence will generally contain ringing but not any decay signals. Various signal processing or statistical methods are applied to remove the estimated ringing component from the acquisition sequences.
U.S. Pat. No. 6,570,381 to Speier et al. discusses using a series of cycles of measurement pulse sequences applied to a formation surround a borehole. Each pulse sequence includes an RF excitation pulse at several RF refocusing pulses. Spin echoes are received that contain spurious ringing signals from the excitation and refocusing pulses. For example, manipulating the polarities of the excitation and refocusing pulses can obtain the substantial cancellation of the spurious ringing from the excitation and refocusing pulses. Spin echo signals from corresponding spin echoes of each cycle are combined and substantially cancel the spurious ringing from the excitation and refocusing pulses of the pulse sequences. U.S. Pat. No. 6,518,757 to Speier discusses rotating the nuclear spins in a pulse sequence. A sequence of refocusing pulses is applied a period of time after termination of the excitation pulse to generate a plurality of echoes. The phase of the refocusing pulses is changed so that pairs of echoes in the echo train have opposite ringing phase. Echoes in the echo train having opposite ringing phase are added to cancel ringing in the echo train. The echo train can then be analyzed for amplitude and/or decay characteristics.
U.S. Pat. No. 6,466,013 to Hawkes et al. discusses a method of both maximizing a signal and minimizing RF power consumption. The timing and duration of the RF pulses are altered from conventional CPMG pulses. In an exemplary embodiment, a refocusing pulse having a spin tip angle less than 180° is applied with carrier phase shifted by typically π/2 radians with respect to the 90° tipping pulse. As a consequence, more of the nuclei originally tipped by 90° are refocused, resulting in larger echoes and reduced power consumption of the tool. An additional forced recovery pulse at the end of an echo train may be used to speed up the acquisition and/or provide a signal for canceling the ringing artefact.
U.S. Pat. No. 6,646,438 to Kruspe et al. discusses a method of acquiring NMR spin echo signal using pulse sequences having more than one interecho spacing By proper selection of the variable TE sequences, a desired resolution may be obtained for all expected components (short, medium, and long) while reducing the required time and the required power.
U.S. Pat. No. 6,204,663 to Prammer discusses a method for suppressing magneto-acoustic artifacts in NMR data using a cycle of pulse sequences characterized by a change in the measurement frequency between pulse sequences. In a preferred embodiment, the frequency change is chosen so that spurious signals induced by the excitation pulse may be significantly reduced by combining NMR signals from corresponding echoes received in response to each measurement frequency.
In the prior art, the PAPS sequence is useful only for reducing the ringing effect from the B-pulse. It would be desirable to have a method of removing ringing from both the A pulse and the B pulse in a situation where the ringing may be changing rapidly during the logging process. The present invention fulfills that need.